#include <iostream>
using namespace std;
double a, b, c, d, ret, rs, n;
double jk, ls;
int main(){
	cin >> a >> b >> c >> d;
	jk = a * 1.0 / b;
	ls = c * 1.0 / d;
	ret = jk;
	rs = jk;
	n = 5000;
	for (int i = 0; i < n; i++){
		rs = rs * (1.0 - ls) * jk;
		ret += rs;
	}
	printf("%.10lf", ret);
	return 0;
}
